The invention relates to a means for mounting spirally wound heatable cathodes in electron guns having two holding posts joined together by an insulator.
Electron guns of this kind are used as sources of high-powered electron beams of the kind used for melting and/or evaporating materials in a vacuum. The block cathode, consisting essentially of a massive circular disk, is generally heated by electron bombardment from a directly heated, spirally bent cathode consisting of wire, and then, on the basis of an accelerating voltage present in the system, emits a beam of electrons.
In a known mounting means of the kind described above, the two ends of the wire cathode are aligned parallel with the axis of the entire gun system and held in cylindrical pieces made of tantalum, which for the purpose of the necessary adjustment of the cathode have excentric bores for the cathode ends. The cylindrical pieces are in turn clamped by cap nuts made of graphite against hollow metal terminal screws which in turn are joined to a common insulator. Each of the assemblies consisting of the cylindrical pieces, the cap nuts and the hollow terminal screws constitutes a so-called support post.
In the known system it is found to be disadvantageous that the excentric adjustment of the terminal ends of the wire cathode does not allow a definite installed position, whether in regard to height adjustment nor in regard to centering. Installation is difficult, and contact problems are encountered, which result in a frequent replacement of the cathodes due to oxidation in the area of the support post, especially the graphite cap nuts. Also, due to the great length of the terminal ends of the cathode, a thermal distortion of the cathode is unavoidable, destroying an adjustment achieved with great effort. After the wire cathode has been heated for the first time, however, straightening it by bending is no longer possible, since the brittle cathode material (tungsten) no longer permits further bending.
The invention is addressed to the problem of improving a mounting means for wire cathodes such that the spirally wound portion of these cathodes will be given an extremely precise position with respect to the beam-forming parts disposed around it, both with regard to its installed height and with regard to its centering. In addition, a perfect, long-lasting transfer of the relatively high heating current is to be possible (low contact resistance).